The invention relates generally to a magnetic resonance imaging (MRI) system and more specifically to generation of radio frequency (RF) pulses for large tip angle excitation and spin refocusing.
MRI is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body. In a MRI system nuclear magnetic moments of a portion of the body or other object to be measured are excited at specific spin precession frequencies, which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons that have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90° or 180°. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The RF magnetic field is generally produced by applying RF pulses to RF coils. In some circumstances, the generated RF magnetic field may not provide accurate spin or rotation of nuclei, for e.g., 75° for a desired spin of 90°. Various RF pulse generation techniques have been utilized to correct for these spin errors. These techniques focus on numerical, iterative corrections to an initial RF pulse which itself is obtained from linear approximation to Bloch equations. However, such corrections usually take significant amount of design time and/or computer resources and are dependent upon the initial estimation. Therefore, it is desirable to determine a method and a system that will address the design time and accuracy of the pulses.